1. Field of the Invention
The present invention relates to an in-plane switching (IPS) mode liquid crystal display device (LCD). More particularly, the present inventor relates to an IPS mode LCD providing an improved image quality and a manufacturing method thereof.
2. Description of the Related Art
The cathode ray tube (CRT) has been the most widely used among display devices to display image information on a screen. However, the CRT has many inconveniences due to its large volume and weight compared with the display area.
With the development of electronic industries, the display device whose usage was limited to a TV Braun tube and so forth, is being used and expanded to, for example, personal computers, notebook computers, wireless terminals, vehicle instrument panels, and electronic display boards. Also, due to the development of information communication technology, since it is possible to transmit large capacity image information, the importance of a next generation display device capable of processing and displaying the large capacity image information gradually increases.
Such next generation display devices are required to have lighter, thinner, shorter and smaller characteristics, a high luminance, a large-sized screen, low power consumption and a low price. Among such next generation display devices, a liquid crystal display device (LCD) is in the limelight.
The LCD exhibits a better resolution than other flat displays, and a fast response rate compared to that of the CRT in implementing a moving picture.
One LCD that has been widely used is a twisted nematic (TN) mode LCD. In the TN mode type LCD, after electrodes are respectively formed on two substrates and liquid crystal directors are aligned twisted by 90°, a driving voltage is applied to the electrodes to drive the liquid crystal directors.
However, the TN mode LCD has a serious drawback of a narrow viewing angle.
Recently, LCDs employing a new mode are being actively researched so as to solve the drawback of the narrow viewing angle of the TN mode. As examples of the new mode, there are an in-plane switching (IPS) mode, an optically compensated birefringence (OCB) mode, etc.
The IPS mode LCD generates a horizontal electric field so as to drive the liquid crystal molecules in a horizontal state with respect to the substrates by forming two electrodes on an identical substrate and applying a voltage between the two electrodes. In other words, the longer axis of the liquid crystal molecule does not stand up with respect to the substrates.
To this end, the IPS mode LCD has a small variation in the birefringence of liquid crystal according to a visual direction and thus has an excellent viewing angle characteristic compared with the TN mode LCD.
Hereinafter, a related art IPS mode LCD will be described in detail with reference to the accompanying drawings.
FIG. 1 is a sectional view of a related art IPS mode LCD.
In FIG. 1, the related art IPS mode LCD is formed by attaching a first substrate 118 and a second substrate 119 facing the first substrate 118, and interposing a liquid crystal layer 130 therebetween. A metal film is first deposited on the first substrate 118 and is patterned to form a plurality of gate lines and a plurality of gate electrodes 109 branched from the respective gate lines and formed at a thin film transistor (TFT) region.
Next, a gate insulating layer 120 is formed on an entire surface of the first substrate including the gate electrode 109, and then a semiconductor layer 115 having an active layer 115a and an ohmic contact layer 115b is formed on a predetermined region of the gate insulating layer 120.
On the gate insulating layer 120, a data line 110 forming a matrix configuration together with the gate line is formed.
In the course of forming the data line 110, source/drain electrodes 116 and 117 of a TFT are formed along with the data line 110.
Also, a common line and a common electrode 113 are formed to be parallel with the gate line 110.
On the entire surface of the first substrate 118 constructed as above, a passivation film 128 is formed.
Thereafter, a pixel electrode 114 is formed to be electrically connected with the drain electrode 117 and be parallel to the data line 110.
On the entire surface of the first substrate 118 constructed as above, a first alignment [orientation] film 129 is formed.
On the other hand, on the second substrate 119, a black matrix 121 for preventing a light from being leaked is formed. A color filter layer consisting of color patterns of red (R), green (G), and blue (B) is formed between the black matrixes 121.
On the color filter layer 122, an overcoat layer 123 for planarizing an upper surface thereof and protecting the underlying color filter layer 122 is formed.
Next, a second alignment film 126 is formed on the overcoat layer 123.
FIGS. 2A and 2B are sectional views illustrating operation states of a related art IPS mode LCD in on/off states.
FIG. 2A illustrates an alignment state of liquid crystal in a liquid crystal layer 211 when no voltage is applied between a common electrode 217 and a pixel electrode 230. In FIG. 2A, since no horizontal electric field is formed, the alignment state of the liquid crystal is not changed.
FIG. 2B illustrates an alignment state of the liquid crystal in the liquid crystal layer 211 when a voltage is applied between the common electrode 217 and the pixel electrode 230. In FIG. 2B, an alignment state of liquid crystal 211a over the common and pixel electrodes 217 and 230 is not changed. On the contrary, since a horizontal electric field K is formed, liquid crystal 211b in a region between the common and pixel electrodes 217 and 230 is aligned in the same direction as the horizontal electric filed K.
That is, the IPS mode LCD has a wider viewing angle due to the horizontally-aligned liquid crystal.
FIG. 3 is a flow diagram illustrating a manufacturing method for a related art IPS mode LCD.
In FIG. 3, upper and lower substrates having a plurality of patterns formed thereon are first manufactured in S100.
In S110, the substrates are cleaned to remove any foreign substance thereon. In S120, a polyimide material is printed on the substrate to form an alignment layer.
In S130, the alignment layer is dried and hardened using the heat of high temperature.
In S140, a surface of the hardened alignment layer is rubbed in one direction.
In S150, an adhesive seal pattern is formed at an edge of the upper substrate at a region except a liquid crystal injection hole and spacers are dispersed on the lower substrate.
In S160, the upper and lower substrates are attached together with an accuracy of several μm so as to prevent light leakage.
In S170, the attached substrate is cut into cells. This cutting process includes a scribing process for forming lines on the upper and lower substrates and a breaking process for dividing the scribed substrate into cells by applying an impact thereon.
In S180, liquid crystal is injected through an injection hole into a gap between the upper and lower substrates cut into cells. The injection hole is sealed to complete a desired LCD.
Here, the physical characteristic of the liquid crystal is changed by a molecular arrangement state thereof, and accordingly there occurs a difference in a response to an external force such as an electric field.
Due to the characteristics of the liquid crystal molecule, a control technique for an arrangement state of the liquid crystal molecule is essential for the study on the physical property of the liquid crystal and the construction of the LCD.
Specifically, a rubbing process for uniformly aligning liquid crystal molecules in one direction is essential for a normal driving of the LCD and a uniform display characteristic thereof.
A related art alignment layer forming process for determining an initial alignment direction of liquid crystal molecules will now be described in detail.
The forming of an alignment layer includes a process of depositing a high polymer thin layer and a process of aligning an alignment layer in one direction.
The alignment layer is made mainly of an organic material of polyimide series and is aligned mainly through a rubbing process.
An organic material of polyimide series is deposited on a substrate and a solvent thereof is volatized at about 60˜80° C. Thereafter, the deposited material is hardened at about 80˜200° C. to form an alignment layer. The alignment layer is rubbed in one direction with a roller having a rubbing cloth such as velvet wound therearound to form an alignment direction thereof.
This rubbing process enables an easy and stable alignment process and is thus suitable for mass production of the LCD.
However, the rubbing process may cause a defect in a rubbing operation when the rubbing cloth becomes defective during the rubbing operation.
That is, the rubbing process is performed through a direct contact between the rubbing cloth and the alignment layer. Therefore, the rubbing process may cause the contamination of a liquid crystal cell due to particles, the damage of a TFT due to an electrostatic discharge, the necessity of an additional cleaning process after the rubbing process, and a non-uniform alignment of liquid crystal in a wide-screen LCD, resulting degradation in a production yield of the LCD.
FIGS. 4A and 4B are, respectively, a sectional view and a plan view illustrating an alignment state of liquid crystal around a step portion of an electrode pattern such as a pixel electrode and a common electrode in a related art IPS mode LCD.
In FIGS. 4A and 4B, an alignment layer 351 is formed on a pixel electrode 330 patterned on a lower substrate and the pixel electrode 330 has a stepped portion with a predetermined step difference.
A color filter layer 360 and an alignment layer 352 are formed on an upper substrate facing the lower substrate and a liquid crystal layer 390 is formed between the upper and lower substrates.
The stepped portion of the pixel electrode 330 causes a non-uniform alignment of liquid crystal in a region therearound.
If the liquid crystal is in a normally-black mode, a black color is displayed when no voltage is applied.
However, there light leakage occurs in a region A shown in FIGS. 4A and 4B when no gate voltage is applied.
That is, when no voltage is applied, the liquid crystal must be aligned in the same direction as the rubbing direction of the alignment layers 351 and 352.
However, the stepped portion of the pixel electrode 330 causes the liquid crystal of a non-uniform liquid layer 391 to have an alignment direction different from a rubbing direction and accordingly causes the liquid crystal of a uniform liquid layer 392 also to have an alignment direction different from the rubbing direction.
The non-uniform liquid crystal causes phase retardation in light. The phase retardation causes a linearly-polarized light to change into an elliptically-polarized light. The elliptically-polarized light causes phase retardation in a uniform liquid crystal layer formed near the color filter layer 360, resulting in a great phase retardation.
Consequently, when no voltage is applied in a normally-black mode, light of the backlight assembly passes through the region A. This causes light leakage in a dark state and a decrease in a contrast ratio, making it difficult to embody a high image quality.
Recently, a super IPS mode LCD for improving a viewing angle and an IPS mode LCD using 3˜4 masks for reducing the number of manufacturing processes are developed and used. In these IPS mode LCDs, the step difference of the stepped portion is increased to cause an increase in an alignment defect.
Accordingly, there is required a device and method for preventing an increase in a black brightness and a contrast ratio due to the stepped portion.